1. Field of the Invention
This invention relates to a method and apparatus for the treatment of molten metals with a gas prior to casting or other processes involving metal cooling and solidification. More particularly, the invention relates to the treatment of molten metals in this way to remove dissolved gases (particularly hydrogen), non-metallic solid inclusions and unwanted metallic impurities prior to cooling and solidification of the metal.
2. Description of the Prior Art
When many molten metals are used for casting and similar processes they must be subjected to a preliminary treatment to remove unwanted components that may adversely affect the physical or chemical properties of the resulting cast product. For example, molten aluminum and aluminum alloys derived from alumina reduction cells or metal holding furnaces usually contain dissolved hydrogen, solid non-metallic inclusions (e.g. TiB.sub.2, aluminum/magnesium oxides, aluminum carbides, etc.) and various reactive elements, e.g. alkali and alkaline earth metals. The dissolved hydrogen comes out of solution as the metal cools and forms unwanted porosity in the product. Non-metallic solid inclusions reduce metal cleanliness and the reactive elements and inclusions create unwanted metal characteristics.
These undesirable components are normally removed from molten metals by introducing a gas below the metal surface by means of gas injectors. As the resulting gas bubbles rise through the mass of molten metal, they adsorb gases dissolved in the metal and remove them from the melt. In addition, non-metallic solid particles are swept to the surface by a flotation effect created by the bubbles and can be skimmed off. If the gas used for this purpose is reactive with contained metallic impurities, the elements may be converted to compounds by chemical reaction and removed from the melt in the same way as the contained solids or by liquid-liquid separation.
This process is often referred to as "metal degassing", although it will be appreciated from the above description that it may be used for more than just degassing of the metal. The process is typically carried out in one of two ways: in the furnace, normally using one or more static gas injection tubes; or in-line, by passing the metal through a box situated in the trough normally provided between a holding furnace and the casting machine so that more effective gas injectors can be used. In the first case, the process is inefficient and time consuming because large gas bubbles are generated, leading to poor gas/metal contact, poor metal stirring and high surface turbulence and splashing. Dross formation and metal loss result from the resulting surface turbulence, and poor metal stirring results in some untreated metal. The second method (as used in various currently available units) is more effective at introducing and using the gas. This is in part because the in-line method operates as a continuous process rather than a batch process.
For in-line treatments to work efficiently, the gas bubbles must be in contact with the melt for a suitable period of time and this is achieved by providing a suitable depth of molten metal above the point of injection of the gas and by providing a means of breaking up the gas into smaller bubbles and dispersing the smaller bubbles more effectively through the volume of the metal, for example by means of rotating dispersers or other mechanical or non-mechanical devices. Residence times in excess of 200 seconds and often in excess of 300 seconds are required in degassers of this type to achieve adequate results. Effectiveness is frequently defined in terms of the hydrogen degassing reaction for aluminum alloys and adequate reaction is generally considered to be at least 50% hydrogen removal (typically 50 to 60%). This results in the need for deep treatment boxes of large volume (often holding three or more tons of metal) which are unfortunately not self-draining when the metal treatment process is terminated. This in turn gives rise to operational problems and the generation of waste because metal remains in the treatment boxes when the casting process is stopped for any reason and solidifies in the boxes if not removed or kept molten by heaters. Moreover, if the metals or alloys being treated are changed from time to time, the reservoir of a former metal or alloy in a box (unless it can be tipped and emptied) undesirably affects the composition of the next metal or alloy passed through the box until the reservoir of the former metal is depleted. Various conventional treatment boxes are in use, but these require bulky and expensive equipment to overcome these problems, e.g. by making the box tiltable to remove the metal and/or by providing heaters to keep the metal molten. As a consequence, the conventional equipment is expensive and occupies considerable space in the metal treatment facility. Processes and equipment of this type are described, for example, in U.S. Pat. Nos. 3,839,019 and 3,849,110 to Bruno et al; U.S. Pat. Nos. 3,743,263 and 3,870,511 to Szekeley; U.S. Pat. No. 4,426,068 to Gimond et al; and U.S. Pat. No. 4,443,004 to Hicter et al. Modem degassers of this type generally use less than one litre of gas per kilogram (Kg) of metal treated. In spite of extensive development of dispersers to achieve greater mixing efficiency, such equipment remains large, with metal contents of at least 0.4 m.sup.3 and frequently 1.5 m.sup.3 or more being required. One or more dispersers such as the rotary dispersers previously mentioned may be used, but for effective degassing, at least 0.4 m.sup.3 of metal must surround each disperser during operation.
To avoid problems associated with deep treatment boxes, there have been a number of attempts at metal treatment in shallow vessels such as the trough provided between the metal holding furnace and the casting machine. This would provide a vessel which could drain completely after use and thus avoid some of the problems associated with the deep box treatment units. The difficulty is that this would inevitably require a reduction of the metal depth above the point of gas injection while still allowing for effective gas/metal contact times. The use of gas diffusion plates or similar devices in the bottom of such shallow vessels or troughs has been proposed to introduce the gas and create the desired gas/metal contact. These are described, for example, in U.S. Pat. No. 4,290,590 to Montgrain and U.S. Pat. No. 4,714,494 to Eckert. However, bubbles produced in this way still tend to be too large and, given the reduced metal depth, such vessels or troughs necessarily must be made undesirably long to achieve effective degassing, and the volume of gas introduced must be made quite high (typically over 2 litres/Kg). As a result, the apparatus takes up a lot of floor space and the volume of gas introduced creates a risk of chilling the metal so that it may be necessary to provide compensating heaters. Such trough degassers can be drained, but because of large bubble size they still require long residence times to effectively treat metal to the same degree of efficiency as obtained with other in-line methods. In addition, the introduction of large gas bubbles into a shallow metal volume results in excess surface turbulence and splashing. As a result, degassing in shallow troughs is not generally carried out on an industrial scale.
Thus there is a need for a metal treatment method and apparatus that provides effective treatment in short time periods, with correspondingly small volumes of metal, and with low gas consumption. Such processes and equipment would then be able to be carried out in metal delivery troughs with all the advantages of such devices that were noted above, but without the problems of high gas consumption or the space limitations noted.